Testicles. Specifically, the resistance they provide during a pat-down of the crotchal region by a TSA agent-turned-baggage handler.

"We have to search up your thighs and between your legs until we meet resistance," he explained.
"Resistance?" I asked.
"Your testicles," he explained.
"That's funny," I said, "because 'The Resistance' is the actual name I've given to my testicles."
- Jeffrey Goldberg, The Atlantic